cpu/native: fix build with afl-gcc 11.2
cpu/native: fix build with afl-gcc 11.2

Building `fuzzing/gcoap` with afl-gcc 11.2 gives

/home/benpicco/dev/RIOT/cpu/native/native_cpu.c: In function ‘thread_stack_init’:
/home/benpicco/dev/RIOT/cpu/native/native_cpu.c:120:11: error: variable ‘stk’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Werror=clobbered]
  120 |     char *stk = NULL;
      |           ^~~
/home/benpicco/dev/RIOT/cpu/native/native_cpu.c:118:72: error: argument ‘stack_start’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Werror=clobbered]
  118 | char *thread_stack_init(thread_task_func_t task_func, void *arg, void *stack_start, int stacksize)
      |

We can re-write the function to not use this temporary variable and the error goes away.

jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,ant9000/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kaspar030/RIOT,miri64/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,ant9000/RIOT,ant9000/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OlegHahm/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,ant9000/RIOT,miri64/RIOT,miri64/RIOT